Use <fcntl.h> instead of <sys/fcntl.h>

Use <fcntl.h> instead of <sys/fcntl.h>

When building on musl, we get:

```
In file included from tests/zfs-tests/cmd/getversion.c:22:
/usr/include/sys/fcntl.h:1:2: error: #warning redirecting incorrect
 #include <sys/fcntl.h> to <fcntl.h> [-Werror=cpp]
 1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>

In file included from module/os/linux/zfs/vdev_file.c:36:
/usr/include/sys/fcntl.h:1:2: error: #warning redirecting incorrect
 #include <sys/fcntl.h> to <fcntl.h> [-Werror=cpp]
 1 | #warning redirecting incorrect #include <sys/fcntl.h> to <fcntl.h>
```

Bug: https://bugs.gentoo.org/925235
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Sam James <sam@gentoo.org>
Closes #15925